THE APPENDICULATE PHYLUM 37 



material of the body from the coelom and pass it to 

 the exterior. Although they are now in very intimate 

 connection with the coelom they are not developed 

 from its walls : they arise in the skin of the embryo 

 and grow inwards into the coelomic cavity, being thus 

 developed in exactly the opposite way to the coelomo- 

 ducts which are developed from the coelomic walls 

 and grow outwards to the skin. 



Now it happens that although the nephridia are 

 the true excretory organs of the Annelid, yet in these 

 and in other animals the coelomoducts, which are 

 originally genital ducts, may lose their primitive 

 function and take on the function of excretory organs. 

 In animals, therefore, we have to distinguish between 

 these two types of excretory organs which may 

 completely resemble one another in structure, the 

 nephridia or primitive excretory organs which are 

 developed from the exterior and push into the 

 coelom, and the coelomoducts^ which are primitively 

 genital ducts and secondarily excretory organs, which 

 develop from the coelomic wall and push to the 

 exterior. 



Let us now turn to our comparison with the 

 Arthropod, Insect, Arachnid or Crustacean, which 

 we have dissected. On opening such an animal we 

 shall at once perceive that there is no spacious seg- 

 mented body-cavity, but the various organs lie in 

 an irregular system of spaces filled with a colourless 



